[Haskell-cafe] Monads as control structures?

[ChrisK]

Try this:

This line is before the loop
sequence_ $ replicate 10 $ do
  line 1
  line 2
  ...
  last line
This line is after the loop


Now you can use shorthand via

loopN n block = sequence_ $ replicate n block

So that you can write:

This line is before the loop
loopN 10 $ do
  line 1
  line 2
  ...
  last line
This line is after the loop

If you need a loop parameter "mapM/mapM_ block [1...10]" works well.



Creighton Hogg wrote:
> Hi,
> so I'm a newbie getting used to Haskell.  I'm writing some 
> simple things like genetic algorithms in it for practice, 
> and I keep coming across something that really bugs me:  
> are there any standard libraries that allow you to 
> do imperative style for or while loops using monads to keep 
> track of state?
> 
> I know there's things like "until", but honestly that's not 
> quite what I'm looking for.  
> 
> I just think there should be a simple way to say "execute 
> this block of code 10 times" without having to wrap it up in 
> recursion.  
> 
> Haskell seems to me to be a very powerful language, and it 
> looks like it should be possible to define control 
> structures such as for loops using monads.
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